Extrajunctional acetylcholine receptors on myogenously de=innervated muscle fibers.

EXTRAJUNCTIONAL ACETYLCHOLINE RECEPTORS ON MYOGENOUSLY DE-INNERVATED MUSCLE FIBERS Di ffuse extrajunct ional acetylcholine receptor (AChR) throughout the sarcolemma has been demon-strable in denervated (neurogenously de-innervated) skeletal muscle fibers pharmacologically by acetyl-choline sensitivity (1, 8) and morphologically by the binding of a-bungarotoxin (aBT) (5, 7, 9), in contrast to the strict confinement of AChR to the neuromus-cular junction in normally innervated muscle fibers. With an immunoperoxidase technique (2) the extra-junctional aBT binding was precisely localized to the sarcolemma membrane complex in neurogenously de-innervated muscle fibers at light and electronmi-croscopic level (9), which led to the hope of using the presence of diffuse extrajunctional AChR as a method for specifically identifying neurogenic disease processes. However, pharmacologic studies have indicated that diffuse extrajunctional AChR is present in developing muscle fibers before neural influence is established on them during embryogenesis (3) and in the surviving portions of muscle fibers distal to a narrow crush lesion separating them from their motor end-plate zone of innervation (6). Considered together, these findings point toward a general principle that all muscle fibers noninnervated or de-innervated by neurogenous or myogenous mechanisms can bear diffuse extrajunctional sarcolemmal AChRs. If correct , this would indicate that extrajunctional AChR has a broad spectrum of usefulness for demonstrating inadequately innervated muscle fibers but, by the same token, should not be considered as distinguishing between neurogenous and myogenous dysinner-vated states or primary noninnervated states. To clarify further this principle and provide visuali-zation of extrajunctional AChR in a myogenously dysinnervated model, a central narrow crush lesion of the sternohyoid muscle was performed in anesthe-sized male rats just distal to the motor endplate zone of innervation, according to the method of Hall-Craggs (4). This allowed examination of a proximal innervated segment and of an apparently viable, structt .irally intact, nondegenerating distal muscle fiber segment parted from neural influence myopath-ically. Fresh-frozen muscle were evaluated 1-17 days thereafter by light-microscopy for the presence of extrajunctional AChR by the aBT-immunoperoxidase technique (2, 9). Serial sections were also stained with the modified trichrome, NADH-tetrazolium reduc-tase, myofibrillar ATPase, nonspecific esterase and alkaline phosphatase reactions. The " recovering " original fiber segments distal to the crush were essentially normal, although an occasional fiber had an internal nucleus. In spite of this, they all had extrajunctional AChR diffusely throughout the sarcolemma as early as 4 days (Fig. 1A). The B _________ FIG. 1. Rat sternohyoid muscle fibers, fresh-frozen, longitudinal sections, 7 days after …